1. Field of the Invention
The present invention relates generally to power converters, and more particularly, to flyback power converters.
2. Description of the Related Art
A dual-switch flyback power converter is a highly efficient power converter since the leakage inductance's energy of its transformer can be retrieved to its input power rail. A patent titled “Highly Efficient Switching Power Converter Using a Charge Pump to Power the Driving Circuit” by Yang, U.S. Pat. No. 7,460,380 has been disclosed recently. In this prior art, a charge pump circuit composed of a diode 80 and a capacitor 85 is activated by a transistor 31. The charge pump circuit is utilized to power a high-side driving circuit 90. Despite the switching signal S1 driving the high-side transistor 11 via the high-side driving circuit 90 and driving the low-side transistor 21 via the low-side driving circuit 95, another switching signal S2 is required to control the transistor 31 to activate the charge pump circuit. However, the duty cycle of the switching signal S2 is limited by the feedback signal VFB from the secondary circuit 60. Once the duty cycle of the transistor 31 is not near 100%, the capacitor 85 may be unable to be charged to maintain a sufficient voltage level to power the high-side driving circuit 90.